1. Field of the Invention
The present invention relates to a semiconductor storage device comprising a plurality of memory elements, and a mobile electronic apparatus comprising the semiconductor storage device. More particularly, the present invention relates to a semiconductor storage device, in which multiple bits (2 bits or more) of data can be stored in at least one of two memory function sections contained in a memory element, and a mobile electronic apparatus comprising the semiconductor storage device.
2. Description of the Related Art
One representative type of conventional non-volatile memory is the flash memory.
In the flash memory, as shown in FIG. 30, a floating gate 902, an Insulating film 907, and a word line (control gate) 903 are provided via a gate insulating film on a semiconductor substrate 901 in this order, and a source line 904 and a bit line 905, which are diffusion regions, are provided on opposite sides of the floating gate 902, constituting a memory cell. The memory cell is surrounded by an isolation region 906 (e.g., Japanese Laid-Open Publication No. 5-304277).
The memory cell holds data depending on the more or less of the amount of charge in the floating gate 902. A plurality of memory cells are arranged into a memory cell array. In the memory array, data is rewritten into or read from a desired memory cell by applying a prescribed voltage thereto via a particular word line and bit line selected.
Such a flash memory has drain current (Id) vs. gate voltage (Vg) characteristics as shown in FIG. 31 when the charge amount of the floating gate is changed. When the amount of negative charge in the floating gate is increased, the threshold is increased and the Id-Vg curve is substantially translated in a direction such that Vg is increased.
However, in this flash memory, the insulating film 907 is functionally required to separate the floating gate 902 from the word line 903. In addition, it is difficult to obtain a thin gate insulating film, because the thinner film is likely to cause charge leakage from the floating gate 902. Therefore, it is difficult to practically obtain a thin insulating film 907 and a thin gate insulating film, which is an obstacle to the miniaturization of memory ealls.